more work towards IEC bus support: Work on LOAD routine and the necessary IEC functions for it.

c64/basic/cmd_unimplemented.s

@@ -84,8 +84,8 @@ cmd_unimplemented:
 
 	;; Dummy action while testing IEC support
 
-	;; Try to open an IEC file
+	;; Try to load an IEC file
-	jsr $ffc0
+	jsr $ffd5
 	bcc +
 
 	;; A = KERNAL error code, which also almost match

c64/kernal/iec_release_atn.s

@@ -0,0 +1,5 @@
+iec_release_atn:
+	lda $dd00
+	and #$f7
+	sta $dd0
+	rts

c64/kernal/iec_turnaround_to_listen.s

@@ -0,0 +1,17 @@
+	;; 	According to http://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
+	;;  	pages 7--8.
+	
+iec_turnaround_to_listen:
+	;; Assert data and release clock
+	lda $dd00
+	and #$ef
+	ora #$08
+	sta $dd00
+
+	;; Wait for clock line to be asserted by the drive
+*	lda $dd00
+	and #$10
+	beq -
+	
+	rts
+	

c64/kernal/load.s

@@ -1,3 +1,86 @@
-; Function defined on pp272-273 of C64 Programmers Reference Guide
+				; Function defined on pp272-273 of C64 Programmers Reference Guide
+	;; IEC reference at http://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
 load:
+
+	;; Disable IRQs, since timing matters!
+	SEI
+
+	;; Begin sending under attention
+	jsr iec_assert_atn
+
+	;; XXX - Use default device number
+	;; http://www.zimmers.net/anonftp/pub/cbm/programming/serial-bus.pdf
+	;; p13, 16.
+	;; also p16 tells us this routine doesn't mess with the file table in the C64,
+	;; only in the drive.
+	
+	;; Call device to LISTEN (p16)
+	lda #$28
+	jsr iec_tx_byte
+	bcs load_error
+
+	;; Open channel #0 (p16)
+	lda #$f0
+	jsr iec_tx_byte
+	bcs load_error
+
+	;; (p16)
+	jsr iec_release_atn
+
+	;; Send filename (p16)
+	lda #$24 		; $ = directory
+	jsr iec_tx_byte
+
+	;; Command device to unlisten to indicate end of file name. (p16)
+	jsr iec_assert_atn
+	lda #$3f
+	jsr iec_tx_byte
+	bcs load_error
+	jsr iec_release_atn
+	
+	;; Now command device to talk (p16)
+	jsr iec_assert_atn
+	lda #$48
+	jsr iec_tx_byte
+	bcs load_error
+	lda #$60 ; open channel / data (p3) , required according to p13
+	jsr iec_tx_byte
+	bcs load_error
+	jsr iec_release_atn
+
+	;; We are currently talker, so do the IEC turn around so that we
+	;; are the listener (p16)
+	jsr iec_turnaround_to_listen
+	bcs load_error
+	
+	;; We are now ready to receive bytes
+	;; jsr iec_rx_byte
+	;; bcs load_done
+
+
+load_done:
+	;; Close file on drive
+
+
+	;; Command drive to listen and to close the file
+	jsr iec_assert_atn
+	lda #$28
+	jsr iec_tx_byte
+	lda #$e0
+	jsr iec_tx_byte
+	jsr iec_release_atn
+
+	;; Tell drive to unlisten
+	jsr iec_assert_atn
+	lda #$3f
+	jsr iec_tx_byte
+	jsr iec_release_atn
+	
+	;;  FALL THROUGH
+	
+load_error:
+	;; Re-enable interrupts and return
+	cli
+	;; (iec_tx_byte will have set/cleared C flag and put result code
+	;; in A if it was an error).
 	rts